Mold Design
TECH 4571
Plastics Molding & Forming Processes
•Compression molding
•Transfer molding
•Injection molding
•Extrusion
•Rotational molding
•Blow molding
•Thermoforming
•RIM
•Casting
•Forging
•Foam molding
•Vacuum molding
•Pultrusion
•Calendering
Example: 96 cavity, 4 sec. cycle time
Design Considerations
•KEY PARAMETERS
–End Use (Start with the end in mind)
–Part Size
–Number of parts to be produced
–Tooling cost
–Family molds
Standard Parts
Ease or difficulty of design changes
http://upload.wikimedia.org/wikipedia/commons/4/42/Standard_two_plate_injection_molding_tool.jpg
Design Considerations for Molding Plastics
–Tolerances
–Mold shrinkage
–Draft or cavity wall taper
–Wall thickness
–Weld Lines
–Finished part appearance
–Quality control
–Cost
http://www.flickr.com/photos/core-materials/3840240677/in/photostream
Weld Line Failure
Design Considerations for Molding Plastics
Features
–Fillet Radii (as large as possible)
–Ribs (increase strength without weight)
•Width (no larger than ½ to ¾ the thickness of the wall to which it is attached)
•Height (no larger than 3x the thickness of the wall to which they are attached)
See: EDMing ribs
–Taper (draft) ½ to 3 degrees per side
Design Considerations for Molding Plastics
Holes – some holes may need to be drilled.
–Distance from hole to edge no less than 3x hole diameter
–Minimum distance between holes = 5x hole diameter
–Hole size should be 120% to 140% of the fastener or shaft O.D. to avoid thermal stress problems
–Threads
•Either class 1 or class 2 fit.
•External or internal (removable threaded pin)
Or design for inserts/overmolding
Holes may introduce weld lines where the material must part and rejoin itself (a built-in weakness)
Draft
•1/2 to 3 degree is recommended
•Highly polished surface allows a small draft
•For textured side walls, additional draft is needed
Undesirable Properties of Plastics
Large coefficient of thermal expansion
Low thermal conductivity
Creep
Shrinkage and warpage (anisotropic)
Fatigue
Injection Mold Design Considerations
•Mold Machine
•Mold Construction
•Multi-Cavity Molds
•Cold Runner Systems
•Hot Manifold/Runnerless
•Gate Design
•Mold Cooling
•Ejection System
•Tooling Material Selection
•Surface Finish
•Venting
Mold Nomenclature
A and B Sides
The A side (cavity side) is fixed and contains the sprue bushing.
The B side (core side) moves back and forth to open the mold. It houses the ejection pins.
The part should stick to the B side.
Halves are aligned with leader pins, interlocks or “locks”
Injection Mold Nomenclature
A and B Sides, Cavity and Core
Gates
Gates are the features that allow molten plastic to flow from the runner into the mold cavity. The type and location of the gate depends on the geometry and end use of the part.
Gate Design Influences
Mold filling pattern
Weld or knit quality and location
Degree and direction of orientation
Dimensional tolerances
Ability to control sink marks
Ability to control shrinkage voids
Pressure distribution in the cavity
Gas traps and short shots
Ease of degating
Residual Stress
Warpage and Flatness
Mallory, Plastic Part Design for Injection Molding pp. 15
Side Gate
Most common gate type. Used for molds with two or more cavities. These gates are removed manually.
Direct Gate
Sprue is the gate. Low cost, simple.
euramould.com
Submarine/Tunnel Gate
Useful for placing a gate anywhere along a product's side wall. Can be automatically cut off by opening mold halves. Available as insert.
See (pdf)
Or sub gate, cashew or banana gate
Submarine/Tunnel Gate
Fan Gate
Used for large flat parts, placed along an edge. Removed manually. Generally for flat panel or box products.
Film Gate
Similar to a fan gate, but thinner.
Influence of Gate Location on Flow
Ideally, the material should flow through the part evenly with no weld lines. Consider how differently the material would flow through each of the following gate locations.
Example: Molded Gear
It is common for plastic gears to be gated with multiple pins on the gear face.
Sprues
3-5 Degree Tapered, polished hole.
Feeds plastic into the runners.
Sprue bushings (like many mold components) can be purchased as standard parts. They are ground as necessary to match the parting line.
Sprue bushing mates with injector nozzle.
May be heated.
Sits inside a locating ring.
Sprue Bushing
Locating Ring
Guides injector nozzle to sprue bushing.
Sprue Puller
The sprue puller is built into the B side of the mold. It is tapered so it pulls the solidified sprue out of the sprue bushing.
It is then ejected out with an ejector pin.
Runners
The best runner profile has a round cross section.
Often only one half is machined.
Some molds are "hot runner" molds.
These use electric heating elements to keep the material molten.
Another mold type is an insulated runner mold.
These rely on the insulating property of the material to keep the runner molten.
Runners - Automatic De-gating
Hot Runner Mold
Insulated Runner Mold
Cold Runner 3-plate mold
2 Plate mold with Tunnel Gates
3 Plate Mold
Hot Runner Advantages
•Production increase (reduced cycle time)
•Material saving
•Quality improvement
•No waste
•Automatic degating
•Energy saving
Flexible choice of gate location
Hot Runner Mold Disadvantages
•More complex mold design, manufacture, operation, and maintenance
•Higher cost
•Thermal expansion of various components must be taken into account
Runner Design
Runner distance should be balanced among the mold cavities.
Consider the following designs:
(Unbalanced Design)
Notice excessive flash around the gate
https://www.reddit.com/r/InjectionMolding/comments/1mtf1zj/cable_tie/
Runner Design
Branched Runners
Each time a runner branches, it should reduce in diameter.
Runner intersections should include a cold slug well.
Mold Components
Image: DSM Design Guide: Performance and Value with Engineering Plastics
Notes about the Mold Models
Coring & Slides
Slide
Core
This core has passages for core slides and ejection pins.
Core Slides
Ejection Pins
Ejector Pin Retainer Plate
Core Slide Pullers
Ejector Plate
Ejector Rod, Guide Pins & Bushings
Back Plate (1), Spacer Block (2), and Support Plate (3)
1
2
3
Alignment Lock (1), Springs (2), Return Pins (3)
1
2
3
Cavity Insert
Cavity Plate (1), Sprue Bushing (2), Locating Ring (3), Clamp Plate (4), Insulation Plate (5)
1
2
4
3
5
Assembly
Mold Components
Mold Components
Image: DSM Design Guide: Performance and Value with Engineering Plastics
Coring
•Used to create hollow areas.
•Ideally, should be parallel to the line of draw.
•Other directions require cams or hydraulic actuators.
Collapsible Cores
Slide actuators for cores and undercuts
Core Pull Mechanisms
Segmented Core
Example
Hot Runner System (and gates)
Allow molten plastic to remain in the sprue and runners. The gates are automatically trimmed by the opening mold halves.
Vents
•Vents are used to allow air to escape. Typical vents are .002 thick and about .125 wide.
•Design vents on the “steel safe” side.
•Sintered Vents are available as inserts
Improper venting can result in burns and short shots.
Vents
Ejector Pins and Plate
•Ejection is done by knockout or ejector pins, sleeves, or stripper plates supported on the ejection bar.
Cooling Channels
Cooling affects the mold process and product geometry.
Mechanical properties
Shrinkage behavior
Warpage
Surface Quality
Cycle time
Flow length in thin walled parts
Cooling Channels
Should provide balanced, uniform temperature within the mold cavity.
This does not mean they should be evenly spaced, but spaced evenly around the cavity.
Conformal Cooling by 3D Printing
General Guidelines for Injection Molding Design
Avoid sharp corners, especially sharp changes in wall thickness.
Avoid thick parts.
Maintain a consistent wall thickness.
Design ribs, etc. to provide balanced, even flow of molten plastic. (for example: more, smaller ribs are preferable to fewer, thicker ribs.
Avoid slides and undercuts. Use draw direction instead of slides where possible.
General Guidelines for Injection Molding Design
Design with the parting line in mind.
In multiple cavity molds, design with balanced feed system. Path to part should be the same for each cavity.
SPI Mold Finish Designations
SPI Finish Guide Typical Applications
A-1 Grade #3 Diamond Lens / Mirror – requires 420 SS material
A-2 Grade #6 Diamond High Polish parts
A-3 Grade #15 Diamond High Polish parts
B-1 600 Grit paper Medium Polish parts
B-2 400 Grit paper Medium Polish
B-3 320 Grit paper Med – Low polish
C-1 600 Stone Low Polish parts
C-2 400 Stone Low Polish parts
C-3 320 Stone Low Polish parts
D-1 Dry Blast Glass Bead Satin finish
D-2 Dry Blast # 240 Oxide Dull Finish
D-3 Dry Blast # 24 Oxide Dull finish
(photo)
Flash Trimming Dies
Mold Manufacturing (video)
Rapid Tooling
See
Spark Mold (YouTube)
https://www.keyence.com/ss/products/measure-sys/machining/formula/injection-molding.jsp#:~:text=Molding%20Shrinkage%20Calculation%20(s),-lc%20(mm)%3A&text=The%20difference%20between%20the%20dimensions,by%20the%20molding%20shrinkage%20ratio.&text=100%20%C3%97%20(400%20%2D%20399)%20%C3%B7%20400%20%3D%200.25(%25).